A generic gas inlet element is already known from EP 1 252 363 B1 and from EP 0 687 749 A1. The known gas inlet elements have a number of chambers arranged one above the other, which are respectively connected by a multiplicity of channels to the chamber bottom, so that the various process gases that are introduced into the chambers can exit from the bottom of the gas inlet element out of the openings in a distributed manner. The gases thereby enter a process chamber disposed underneath the bottom of the gas inlet element, the bottom of which is formed by a substrate holder on which there lie one or more substrates, which are coated by means of the components introduced into the process chamber by the process gases. The known gas inlet elements are used for MOCVD.
Metal Organic Chemical Vapor Deposition (MOCVD) is a widely used method for depositing single- or multi-component oxidic insulating layers (dielectrics), semiconductor layers, passivation layers or electrically conducting layers. For this purpose, a number of reactive gases or gaseous precursors are mixed, fed to a reaction chamber in order to deposit a layer on a heated substrate, and then pumped out from the reaction chamber. Among the reactors there are various geometric forms, for example horizontal and vertical reactors. In the case of horizontal reactors, the substrate surface is parallel to the direction of flow of the mixed precursors and reactive gases. In vertical reactors, the corresponding gas mixture impinges vertically onto the substrate surface and flows off to the outer edges of the substrate before it leaves the reaction chamber. In general, rotation of the substrate can lead to an increase in the uniformity of the layer that is deposited.
In order to ensure homogeneous deposition on the substrate, thorough mixing of the various gaseous precursors or reactive gases must be ensured. In order to achieve this, there are methods by which the gas mixing is achieved at an early stage before introduction into the reaction chamber. This is suitable for precursors and reactive gases that are stable at the temperature and pressure in the gas distributor.
However, the precursors used are often very reactive: the byproducts produced in the gas phase reactions that occur lead to deposition on, and consequently progressive contamination of, gas-carrying parts upstream of the substrate, cause particle formation, and consequently particle coating of the substrate, change the reaction mechanisms on the substrate and reduce the efficiency of the growth process.
In the case of the known multi-chamber gas distributor (cf. also U.S. Pat. No. 5,871,586), the various gaseous components are supplied in separate chambers and fed directly to the substrate via a multiplicity of openings. The mixing is effected only in the region directly at the substrate. In the case of some processes for oxidic insulating layers (dielectrics), passivation layers or electrically conducting layers, it has been found that this type of mixing does not lead to sufficiently homogeneous layers on the substrate. For some applications, the requirement for the inhomogeneity of the deposited layers on the substrate surface is, for example, <+−1%.
Many gaseous metal-organic precursors are only stable as such within a small temperature range. Metal-organic precursors may contain at least one metal atom and/or also at least one semiconductor atom (such as for example Si, Ge). At temperatures that are too low, condensation takes place, at temperatures that are too high decomposition takes place even before mixing with other reactive gases. It is therefore necessary to keep the gas distributor at a homogeneous temperature.
It is an object of the invention to provide measures that improve the way in which a generic gas inlet element operates.
The object is achieved by the invention specified in the claims, each of the claims in principle respectively representing an independent solution and it being possible in principle for each claim to be combined with any other claim or with any feature of any other claim.
Claim 1 provides first and foremost that the access openings are preceded by at least one mixing chamber arrangement, in which at least two process gases are mixed with one another. As a result of this configuration, the gases are already mixed when they are introduced into the central chamber of the gas inlet element, where they can then enter the process chamber from the outlet openings at the bottom in the mixed state. It has been found to be advantageous if the chamber is surrounded by an annular distribution channel. This annular distribution channel has a multiplicity of access openings, which are directed at the central chamber of the gas inlet element and from which the already mixed process gases flow into the central chamber. The central chamber may have a circular disk-shaped form. The annular distribution channel then surrounds the chamber in the form of a circular annulus. In a development, it is provided that the gas flow exiting from the mixing chamber arrangement is directed into the annular distribution channel. For this purpose, gas flow directing means may be provided, which in the simplest case are formed as strips of sheet metal. These protrude into the annular distribution channel in the manner of blades, in order to impart a certain inflow direction to the gas flow. In a preferred development, there are a multiplicity of mixing chamber arrangements, disposed substantially in uniform circumferential distribution. Preferably, a number of mixing chamber arrangements, in particular at least four and preferably six, are connected in uniform angular distribution to the annular distribution channel. These mixing chambers lie radially on the outside. In a development of the invention, it is provided that the mixing chamber arrangement consists of two or more chambers. Supply lines for a respective process gas open out into each of the two chambers. In this case, only one chamber may be connected directly to the annular distribution channel or to the central chamber. The two chambers are preferably separated by means of a gas-permeable separating wall. It is possible, for example, for the second process gas, introduced into the second chamber of the mixing chamber arrangement, to flow through this separating wall into the first chamber of the mixing chamber arrangement, in order then to flow together with the first process gas into the annular distribution channel. The gas-permeable separating wall is preferably formed by a perforated plate, which is exchangeable, so that it can be matched to the respective process taking place in the process chamber. The gas-permeable separating wall is preferably a continuation of the gas flow directing means. It is regarded as advantageous that one or more gaseous precursors and one or more chemically reactive gases are introduced separately into the mixing chamber arrangement and mixed there in at least one mixing chamber arrangement. After leaving the at least one mixing chamber arrangement, the gases flow in a circumferential direction into the annular distribution channel. After that, they flow via openings in a radial direction into the interior space of the gas inlet element. The interior space of the gas inlet element is provided with a multiplicity of outlet openings. From these openings, disposed in the manner of a shower head, the gas mixture flows into the process chamber. Preferably, at least four mixing chamber arrangements are provided. Each mixing chamber has preferably a reaction gas chamber, into which a chemically reactive gas is introduced. It also has at least one precursor chamber, into which at least one gaseous precursor is introduced. The gas outlet from the mixing chamber arrangement into the annular distribution channel is effected preferably via the precursor chamber. The precursor may contain a metallic component, the reactive gas oxygen or nitrogen. The gases exit from the outlet openings in the bottom of the central chamber homogeneously. The annular distribution channel may be formed by an annular groove in a metal plate, which is provided with a multiplicity of recesses which form the central chamber and the individual chambers of the mixing chamber arrangements. The temperature of the gas distributor can also be controlled. The coating process operated in the process chamber preferably takes place at a process pressure of from 0.001 Pa to 5 bar. The openings within the gas inlet element are formed such that they do not have any dead volume. Therefore, the chambers of the mixing chamber arrangement also have rounded walls. The dwell time of the gases in the respective mixing chamber and up to the annular distribution channel is less by a multiple than the dwell time of the gases in the central chamber of the gas inlet element. At room temperature, the precursors may be liquid or solid metal-organic starting materials or metal-organic liquid or solid starting materials that are dissolved in liquid or solid solvents. These are vaporized for conversion into the gas phase. This vapor is then fed to the process chamber via the gas inlet element. This may take place with the aid of one or more inert carrier gases. It is also possible to use precursors that are already in gas form at room temperature and are fed directly to the gas distributor. This may also take place with an inert carrier gas. Oxygen compounds are used in particular as the chemically reactive gas. They may comprise O2, O3, N2O or H2O. However, nitrogen compounds, in particular NH3, may also be used as reactive gases. The chemically reactive gases may also be hydrogen compounds. N2, H2, helium, argon or any other noble gas or inert gas come into consideration in particular as carrier gases. In the CVD reactor, which is equipped with a gas inlet element of this type, preferably multi-component oxidic insulating layers (dielectrics), passivation layers, semiconducting layers or electrically conducting layers and layer sequences are produced on at least one substrate. The precursors (starting materials) preferably comprise a metallic or semiconducting component.